Ihave a 350 chevy engine that I am priming the oil to my rockers on the engine stand and noticed a little oil leaked out from around the back of oil pan or rear main seal.It is anew rear main seal and Iused the one piece fel-pro oil pan gasket. Never had this to happen before.Has anybody else had this problem.  

What year is the block,did you use any kind of sealer or silicone on the gasket.  

Just used silicone on the corners of pan.That was the only place fel pro said to use it.I didnt think that you could get a leak from just priming oil.I thought maybe having pressure on main bearings with out crank spinning that maybe it pushed some oil through main seal.Just never had that to happen before.  

There are two different oilpans for small blocks, early and late model. The late model pan takes the thick seal, and the early pan the thin seal. To determine which pan you have, install the timing cover then the oil pan without the end seals but with the cork gaskets along the sides. Now try to pass a .006 to .010 feeler guage between the cover and the pan at the very top. If the guage won't go, use the thin seal. If it will, use the thick seal. Most kits come with both. Good luck.